I'm looking for a filter that mimics Cokin graduated filters.
Should be adjustable demarcation line up and down.
Should be able to set at percentage of darkening 30%, 50% etc..
Application of different colors, such as tobacco, blue and others.

Should be usable in PhotoShop 7 (I can't use the new Nic programs as they require CS)

What about the gradient tool in Photoshop 7, Don? You can achieve much the same result without a plug-in ... and tweak it all you like.

Haven't done this but once or twice, but the basic method I've used is to create a new transparent layer above the image, apply the gradient to that new layer and tweak the various settings until you get the right combination of transparency and color (subtle additions of color are usually better). Takes a little trial and error, but it should give you what you want.

You could also record this procedure as a macro and save it in your "actions," and keep it available as a ready-made filter for future use.